The Geography consists of three sections, divided among 8 books. Book I is a treatise on cartography, describing the methods used to assemble and arrange Ptolemy's data. From Book II through the beginning of Book VII, a gazetteer provides longitude and latitude values for the world known to the ancient Romans (the "ecumene"). The rest of Book VII provides details on three projections to be used for the construction of a map of the world, varying in complexity and fidelity. Book VIII constitutes an atlas of regional maps. The maps include a recapitulation of some of the values given earlier in the work, which were intended to be used as captions to clarify the map's contents and maintain their accuracy during copying.

Ptolemy's work included a single large and less detailed world map and then separate and more detailed regional maps. The first Greek manuscripts compiled after Maximus Planudes's rediscovery of the text had as many as 64 regional maps.[b] The standard set in Western Europe came to be 26: 10 European maps, 4 African maps, and 12 Asian maps. As early as the 1420s, these canonical maps were complemented by extra-Ptolemaic regional maps depicting, e.g., Scandinavia.

The original treatise by Marinus of Tyre that formed the basis of Ptolemy's Geography has been completely lost. A world map based on Ptolemy was displayed in Augustodunum (Autun, France) in late Roman times.[citation needed]Pappus, writing at Alexandria in the 4th century, produced a commentary on Ptolemy's Geography and used it as the basis of his (now lost) Chorography of the Ecumene.[18] Later imperial writers and mathematicians, however, seem to have restricted themselves to commenting on Ptolemy's text, rather than improving upon it; surviving records actually show decreasing fidelity to real position.[18]

Despite beginning to compile numerous gazetteers of places and coördinates indebted to Ptolemy,[30] Muslim scholars made almost no direct use of Ptolemy's principles in the maps which have survived.[24] Instead, they followed al-Khwārazmī's modifications and the orthogonal projection advocated by Suhrāb's early 10th-century treatise on the Marvels of the Seven Climes to the End of Habitation. Surviving maps from the medieval period were not done according to mathematical principles. The world map from the 11th-century Book of Curiosities is the earliest surviving map of the Muslim or Christian worlds to include a graticule but the copyist seems to have not understood its purpose, starting it from the left using twice the intended scale and then (apparently realizing his mistake) giving up halfway through.[31] Its presence does strongly suggest the existence of earlier, now-lost maps which had been mathematically derived in the manner of Ptolemy,[26] al-Khwārazmi, or Suhrāb. There are surviving reports of such maps.[30]

Ptolemy's Geography was translated from Arabic into Latin at the court of king Roger II of Sicily in the 12th century CE.[32] However, no copy of that translation has survived.

The Greek text of the Geography reached Florence from Constantinople in about 1400 and was translated into Latin by Jacobus Angelus of Scarperia around 1406.[12] The first printed edition with maps, published in 1477 in Bologna, was also be the first printed book with engraved illustrations.[33][34] Many editions followed (more often using woodcut in the early days), some following traditional versions of the maps, and others updating them.[33] An edition printed at Ulm in 1482 was the first one printed north of the Alps. Also in 1482, Francesco Berlinghieri printed the first edition in vernacular Italian.

Edition printed in Ulm in 1482

Ptolemy had mapped the whole world from the Fortunatae Insulae (Cape Verde[35] or Canary Islands) eastward to the eastern shore of the Magnus Sinus. This known portion of the world was comprised within 180 degrees. In his extreme east Ptolemy placed Serica (the Land of Silk), the Sinarum Situs (the Port of the Sinae), and the emporium of Cattigara. On the 1489 map of the world by Henricus Martellus, which was based on Ptolemy’s work, Asia terminated in its southeastern point in a cape, the Cape of Cattigara. Cattigara was understood by Ptolemy to be a port on the Sinus Magnus, or Great Gulf, the actual Gulf of Thailand, at eight and a half degrees north of the Equator, on the coast of Cambodia, which is where he located it in his Canon of Famous Cities. It was the easternmost port reached by shipping trading from the Graeco-Roman world to the lands of the Far East.[36]
In Ptolemy’s later and more well-known Geography, a scribal error was made and Cattigara was located at eight and a half degrees South of the Equator. On Ptolemaic maps, such as that of Martellus, Catigara was located on the easternmost shore of the Mare Indicum, 180 degrees East of the Cape St Vincent at, due to the scribal error, eight and a half degrees South of the Equator.[37]

Catigara is also shown at this location on Martin Waldseemüller’s 1507 world map, which avowedly followed the tradition of Ptolemy. Ptolemy’s information was thereby misinterpreted so that the coast of China, which should have been represented as part of the coast of eastern Asia, was falsely made to represent an eastern shore of the Indian Ocean. As a result, Ptolemy implied more land east of the 180th meridian and an ocean beyond. Marco Polo’s account of his travels in eastern Asia described lands and seaports on an eastern ocean apparently unknown to Ptolemy. Marco Polo’s narrative authorized the extensive additions to the Ptolemaic map shown on the 1492 globe of Martin Behaim. The fact that Ptolemy did not represent an eastern coast of Asia made it admissible for Behaim to extend that continent far to the east. Behaim’s globe placed Marco Polo’s Mangi and Cathay east of Ptolemy’s 180th meridian, and the Great Khan’s capital, Cambaluc (Beijing), on the 41st parallel of latitude at approximately 233 degrees East. Behaim allowed 60 degrees beyond Ptolemy’s 180 degrees for the mainland of Asia and 30 degrees more to the east coast of Cipangu (Japan). Cipangu and the mainland of Asia were thus placed only 90 and 120 degrees, respectively, west of the Canary Islands.

The Codex Seragliensis was used as the base of a new edition of the work in 2006.[11] This new edition was used to "decode" Ptolemy's coordinates of Books 2 and 3 by an interdisciplinary team of TU Berlin, presented in publications in 2010[38] and 2012.[39][40]

Christopher Columbus modified this geography further by using 53⅔ Italian nautical miles as the length of a degree instead of the longer degree of Ptolemy, and by adopting Marinus of Tyre’s longitude of 225 degrees for the east coast of the Magnus Sinus. This resulted in a considerable eastward advancement of the longitudes given by Martin Behaim and other contemporaries of Columbus. By some process Columbus reasoned that the longitudes of eastern Asia and Cipangu respectively were about 270 and 300 degrees east, or 90 and 60 degrees west of the Canary Islands. He said that he had sailed 1100 leagues from the Canaries when he found Cuba in 1492. This was approximately where he thought the coast of eastern Asia would be found. On this basis of calculation he identified Hispaniola with Cipangu, which he had expected to find on the outward voyage at a distance of about 700 leagues from the Canaries. His later voyages resulted in further exploration of Cuba and in the discovery of South and Central America. At first South America, the Mundus Novus (New World) was considered to be a great island of continental proportions; but as a result of his fourth voyage, it was apparently considered to be identical with the great Upper India peninsula (India Superior) represented by Behaim—the Cape of Cattigara. This seems to be the best interpretation of the sketch map made by Alessandro Zorzi on the advice of Bartholomew Columbus (Christopher’s brother) around 1506, which bears an inscription saying that according to the ancient geographer Marinus of Tyre and Christopher Columbus the distance from Cape St Vincent on the coast of Portugal to Cattigara on the peninsula of India Superior was 225 degrees, while according to Ptolemy the same distance was 180 degrees.[41]

Prior to the 16th century knowledge of geography in the Ottoman Empire was limited in scope, with almost no access to the works of earlier Islamic scholars that superseded Ptolemy. His Geography would again be translated and updated with commentary into Arabic under Mehmed II, who commissioned works from Byzantine scholar George Amiroutzes in 1465 and the Florentine humanist Francesco Berlinghieri in 1481.[42][43]

Considering a sample of 80 cities amongst the 6345 listed by Ptolemy, those that are both identifiable and for which we can expect a better distance measurement since they were well known, there is a systematic overestimation of the longitude by a factor 1.428 with a high confidence (coefficient of determination r² = 0.9935). This error produces evident deformations in Ptolemy's world map most apparent for example in the profile of Italy, which is markedly stretched horizontally.

Ptolemy accepted that the known Ecumene spanned 180° of longitude, but instead of accepting Eratosthenes's estimate for the circumference of the Earth of 252,000 stadia, he shrinks it to 180,000 stadia, with a factor of 1.4 between the two figures.

Ptolemy took as location for the longitude 0° the Fortunate Isles which at his times were identified with the Canary Islands. The strange coincidence of the two aforementioned errors may be accounted for if one assumes that this identification was wrong and that at the time of Ptolemy's sources the Fortunate Isles where actually the eastern Antilles. Since Ptolemy could estimate the actual distance to the Canaries, Russo proposes that he purposely shrank the circumference of Earth to accommodate his data and his wrong identification of the Fortunate Isles. This suggests or even proves that the American continent was known in Classical Antiquity.[47]

^Crone, G.R. (Dec 1964). "review of Theatrum Orbis Terrarum. A Series of Atlases in Facsimile". The Geographical Journal. The Royal Geographical Society (with the Institute of British Geographers). 130 (4): 577–578. doi:10.2307/1792324. JSTOR1792324.

Yule, Henry (1915). Henri Cordier, ed. Cathay and the Way Thither: Being a Collection of Medieval Notices of China, Vol I: Preliminary Essay on the Intercourse Between China and the Western Nations Previous to the Discovery of the Cape Route. 1. Hakluyt Society.

Stevenson, Edward Luther. Trans. and ed. 1932. Claudius Ptolemy: The Geography. New York Public Library. Reprint: Dover, 1991. This is the only complete English translation of Ptolemy's most famous work. Unfortunately, it is marred by numerous mistakes (see Diller) and the place names are given in Latinised forms, rather than in the original Greek.

(in Latin)Geography, digitized codex made in Italy between 1460 and 1477, translated to Latin by Jacobus Angelus at Somni. Also known as codex valentinus, it is the oldest manuscript of the codices with maps of Ptolemy with the donis projections.